Purchased a new 5 7 today and noticed what looks like wear and/or rust on the slide (internal surface) after I got home and disassembled it. Has anyone else here seen or heard of this with 5 7s? Someone else suggested this is likely manufacturer residue perhaps anti-sieze compound?

Yesterday I took it back to the shop where I purchased it and they said it was just brass scuffs from testing at FNH. Another shop down the road said basically the same thing. So overall the consensus is saying it is not rust (thankfully). Fired a box of Hornady 40 grain at the range later same day, and well, it performed flawlessly. Not worried like I was, but will monitor. Thank you for the replies! As requested I have attached a link to pic of the "brass scuffs" (taken day of purchase).